Part of object not visible in render view
I had a similar issue. Part of my mesh was rendered in Render View in 3D ViewPort but all showing in Full Render Image (F12) see image below.

I found the solution; simple enough. 
Select the Mesh in Edit Mode, then go to Mesh Menu > Clean Up > Delete Loose. (Clean the Loose Vertices Left after Applying Multier &amp; Sculpting) 

It seemed that I had some loose vertex which were creating a problem. But somehow they are not causing any issues in Full Render F12. Just issue in Render View in 3D View Port.